Three-phase power

The standard commercial service in the United States is 208Y/120 volts, three phase: three live conductors, a neutral and a separate ground. Measure any one phase to neutral and it reads 120 volts. Measure between any two phases and it reads 208.

That second number is where explanations go wrong. 208 is not 120 doubled and it is not 240. The three phases are offset from each other by a third of a cycle, so their voltages combine as vectors rather than by addition: 120 multiplied by the square root of three, which is about 1.732, gives 207.8. The 240 volts in a house comes from a different arrangement, where two legs are exactly opposite and do add.

Three phase is used at events for three reasons. One service feeds a great many 120 volt circuits spread across three legs, so a large lighting or video load can be balanced instead of piling on to one. A balanced load puts almost nothing on the neutral. And delivering the same power at a higher voltage means less current, which means smaller and lighter cable — which matters when it is being wheeled through a loading dock.

The practical consequence for a show is that a service is only as good as its worst leg. A rig that is heavy on one phase will trip that phase while the other two sit idle, and the arithmetic that said the service was ample was averaging.

Corporate event production — owned inventory, our own crew, and a site survey before anything is specified.

• Power distro The portable box that takes one large feed and breaks it into protected circuits — the thing between a building’s power and every piece of equipment on the show.
• Load calculation Working out what the rig actually draws before anybody orders a service — where the common mistake is treating a 20-amp circuit as 20 amps.
• Cam-Lok (camlock) The single-pole connector feeder cable lands on — one conductor per connector, colour-coded by convention rather than by code, and connected ground first.